NOx emission control devices, for example catalysts containing a precious metal and elements such as barium, cesium, and lanthanum, can be used to reduce NOx emissions for lean burn engines, such as direct injection spark ignition engine. These devices store NOx produced during the engine lean operation. Since these devices have a finite NOx storage capacity, it has to be regenerated every once in a while in order to sustain long periods of lean operation. This regeneration can be accomplished with rich engine operation during which the device releases and converts the stored NOx.
These emission control devices can be exposed to environments with high temperature from the engine exhaust gas and also from the exotherm introduced by chemical reactions, such as reduction of stored oxidants. The high temperature exposure over time can reduce oxidant (e.g., O2 and NOx) storage capacity. In addition to temperature effects, sulfur in the engine exhaust gas can form sulfates on the storage sites, thereby reducing the storage capacity. Both the exhaust emissions and the fuel consumption increase as the storage capacity decreases.
One approach to monitoring the effectiveness of an exhaust emission control device used during lean operation to reduce NOx emissions monitors the rich purging time of rich cycles. Such a device is described in JP 08-232644.
The inventors herein have recognized a disadvantage with approaches that simply consider the rich purging time, or only consider the lean NOx storage time of an emission control device. Specifically, simply monitoring the rich purging time results in large deviation of measurement results depending how much NOx was stored in the device, thereby requiring complex algorithms and significant model calibration efforts. Similarly, only monitoring the lean storage time does not take into account variations in NOx releasing/reduction reactions that can change as the catalyst ages.